The present invention relates to a control channel selecting method for mobile stations of a two-way mobile radio communication system which communicate with a different base station for each service zone, such as automobile and portable telephones.
In a mobile radio communication system such as an automobile telephone system, the zone in which a mobile station is allowed to communicate with one base station is limited according to the transmitting power of the radio wave used, and as shown in FIG. 1, for example, in the case of realizing a wide service area, a number of zones Z1, Z2, . . . , which have base stations B1, B2, . . . installed at their centers, are provided and in any zone mobile stations are allowed to be connected to a communication network via the base station of the zone and a switching system 30. The frequency band assigned to the entire mobile radio communication system consists of many equally spaced-apart radio channels, which are divided into control channels that are used for the base stations to specify channels to be used by mobile stations in their service zones and communication channels for communication use. To prevent radio communications in adjacent zones from interfering with each other, radio channels of different frequencies are assigned to the adjacent zone.
Now, assume that the total number of radio channels of the frequency band assigned to the mobile radio communication system is 100 and let the radio channels be represented by ch1, ch2, . . . , ch100, beginning with the lowest frequency channel. Let it be assumed, for example, that the channels ch1, ch2, . . . , ch10 are used as control channels, which are represented by CCh1, CCh2, . . . , CCh10 in FIG. 1. These control channels CCh1, CCh2, . . . , CCh10 are sequentially assigned one by one to the zones Z1, Z2, . . . in such a manner that the control channels in adjacent zones are not the same. Similarly, pluralities of communication channels are sequentially assigned to the zones Z1, Z2, . . . , respectively, in such a manner that the communication channels in adjacent zones are not the same. In the mobile radio communication system to which the present invention is applied, however, a channel, even if assigned as a control channel to a certain zone, is allowed to be used as a communication channel of another or other zones sufficiently remote therefrom not to suffer interference, so as to improve the frequency utilization efficiency. For instance, as shown in FIG. 1, while channels ch4, 5 and 11 through 20 are assigned as communication channels to the zone 1, the channels ch4 and ch5 are used as control channels CCh4 and CCh5 of the zones Z4 and Z5, respectively.
In a service area (including a plurality of zones) of a high traffic volume as in a big city, a control channel of a certain zone is used as one of the communication channels of another zone sufficiently remote therefrom to thereby increase the traffic volume that can be dealt with by the system.
In such a mobile radio communication system, mobile stations M1, M2, . . . each have prestored therein frequencies of all the control channels CCh1 through CCh10 of the entire system and in each mobile station, upon turning on the power, all the control channels CCh1 through CCh10 are sequentially swept to measure their receiving levels, and that one of the control channels which has the maximum level detected above a prescribed value is automatically selected as the control channel in the zone where the mobile station is currently present. For example, a call request from the mobile station M1 is sent via its selected control channel CCh3 to a base station B3, which, in turn, indicates to the mobile station M1 via the control channel CCh3 an idle one of the communication channels ch8, ch9 and ch31 through ch40 assigned to the zone Z3. The mobile station M1 automatically selects the thus specified channel and becomes ready for originating a call.
As mentioned above, upon turning on the power, the mobile station measures the receiving levels of all the control channels CCh1 through CCh10 one after another and selects the control channel to be used (CCh3 in the zone Z3). As long as the power supply is held 0N thereafter and the mobile station remains in an incoming call waiting state, it repeatedly measures the receiving level of the channel being currently received, that is, the control channel CCh3 currently selected as described above. For example, as the mobile station M1 moves into the zone Z1 from the zone Z3 as indicated by the arrow in FIG. 1, the receiving level of the control channel CCh3 lowers. When the receiving level becomes lower than a prescribed value, the mobile station M1 measures again the receiving levels of all the control channels and selects the control channel of the maximum level (CCh1 in the zone Z1) among them. Such a measurement of the receiving level for each control channel requires, for instance, around 100 msec, and accordingly, one second or so is needed to measure the receiving levels of all the control channels.
Assume, for example, that a mobile station M2 staying in the same zone Z3 as the mobile station M1 is in communication over the channel ch8 when the mobile station M1 is to select the control channel CCh3 of the zone Z3 from all the control channels CCh1 through CCh10 as mentioned above. In this instance, the receiving levels of the channels ch8 as well as CCh3 are detected high in the search for the control channel by the mobile station M1; therefore, the method of merely selecting a channel of the maximum level incurs the possibility that the channel ch8 being occupied by the mobile station M2 is selected as the control channel by the mobile station M1. In such a case, the mobile station M1 cannot receive a control signal from the base station even if it waits for the mobile station M2 to end the communication. To avoid this, it is customary in the prior art not only to check the receiving level of the respective control channel but also to make a check to see if the signal that is received over the control channel has the format of a control signal.
It usually takes about one second to decide that a control signal of the normal format could be received. On the other hand, when such a normal control signal has not been received even after a certain elapsed time (two seconds, for instance), it is decided that no control signal has been received. When the received signal is decided not to be the normal control signal, it is judged that the received signal is a communication signal or disturbing signal and other channels of high receiving levels are similarly checked; that is, such channel checking is repeated until an available control channel is found.
As referred to above, a relatively large amount of time is needed for the mobile station to decide that the received signal is not a control signal, and hence under bad environmental conditions it takes much time for the mobile station to turn ON the power supply, select the control channel and then become ready for communication. Moreover, since no incoming call can be received during the control channel selection, the incoming call reception failure rate unavoidably increases. Where the mobile station is a portable telephone, it is designed so that the normal control channel reception takes place intermittently to reduce power dissipation, and a continuous power supply is needed during the control channel selection; hence, the more time the control channel selection takes, the more batteries are consumed and the more frequently they need to be charged.